1. Field of the Invention
The present invention relates to a technology for playing such musical score data as MIDI (Music Instrument Digital Interface) data, and more particularly to a technology for improving the sound quality of played sound.
2. Description of Related Art
The spread of portable communication terminals, such as portable telephone and PHS (Personal Handyphone System) is being promoted recently. Many portable communication terminals today have a music playback function. Typical use of this music playback function is notifying by sound when a telephone call or email is received. Many portable communication terminals today can notify the arrival of a telephone call and the reception of an email to the user, not by an ordinary call up sound, but by a melody sound. Additionally portable communication terminals which can play melody for listening to music are already known.
For portable communication terminals, MIDI, for example, is used as a standard for music playback. MIDI is a technology not for converting sound itself into data, but for converting musical instrument performance information into data. For example, when the instrument is a keyboard, such musical performance operation as “pressing keys on the keyboard with fingers”, “releasing fingers from the keyboard”, “stepping on a pedal”, “removing feet from a pedal” and “changing tone” is converted into data. The musical score data conforming to the MIDI standard is called “MIDI data”. As technology for playing MIDI data, technology stated in Japanese Laid-Open Patent Application Nos. 9(1997)-127951 and 9(1997)-160547, for example, are known.
Musical score data, such as MIDI data, is stored in a portable communication terminal during manufacturing, or is downloaded to a portable communication terminal using communication functions. The service to download musical score data to a portable communication terminal can dramatically increase the choices of a played music, so it is used by many users.
As portable communication terminals having music playback functions spread, the demand for improving the sound quality of played sounds has the tendency to increase. Today a sound quality which satisfies listening to a melody, and not just satisfying the level of notifying by sound, is demanded.
To improve the sound quality, it is desirable to use a high performance speaker. However it is difficult to install a high performance speaker in a portable communication terminal. This is because a portable communication terminal demands not only an improvement in the sound quality but also a decrease in the size and weight of the terminal. Therefore a very small speaker, with less than a 1 centimeter diameter, for example, is installed in a normal portable communication terminal. Small speakers generally have characteristics where the gain (decibel) of a high tone is large and the gain of a low tone is small. Normally, it is difficult to obtain sufficient gain at a 500 Hz or less frequency for a speaker with less than a 1 centimeter diameter.
Also the type of speaker to be installed in a portable communication terminal differs depending on the manufacturer and model of the terminal. Therefore the characteristics of speakers are not same, but differ depending on the manufacturer and model of the terminal.
A method for improving the sound quality of a small speaker is shifting the entire played sound to the high tone side. By this method, the gain of the played sound can be increased, and consequently the user can hear the played sound more easily. This method, however, can improve the usability of a notifying sound, but cannot assure sufficient sound quality in terms of listening to a melody.
Another method for improving the sound quality is using an equalizer. An equalizer is a device for adjusting the frequency characteristics of an acoustic signal. By increasing the amplification factor of an acoustic signal with respect to the low frequency component, the low tone gain of a speaker can be substantially increased. Additionally the dispersion of the sound quality due to the differences of the characteristics of a speaker can be suppressed by changing the equalizer settings according to the type of speaker.
However, it is difficult to install an equalizer in a portable communication terminal, since the terminal size increases and price increases. An equalizer can be configured by software, but it is difficult to use this software in a portable communication terminal. Because a high performance processor must be installed in the portable communication terminal, which increases the size of the device and increases price.
Such problems are not limited to portable communication terminals, but are common to music playback units where a high performance speaker and circuit cannot be installed.